Early bows consisted of a simple stick with a string attached to each end. In 1969, U.S. Pat. No. 3,486,495 (which is incorporated herein by reference) was granted to Holless Allen for a compound bow. By providing eccentric leverage draw force modules on the end of limbs with operating cabling, the force draw curve of the Allen compound bow could be manipulated to store more energy during the draw cycle, firing a faster arrow and lessening the holding weight at full draw, thereby allowing better aiming. As the bowstring was pulled, it was unspooled from the draw force modules, while cabling pulling on the opposing limbs was spooled on the draw force modules.
A problem is that one cam could move independently of another during letoff at the end of the draw. Thus there would be an inconsistent shift in nock travel at the string, anchor and release, resulting in inaccuracy.
Currently bows have become popular with cables from cams attaching to the opposite cams to synchronize the cams. However, a problem has risen with limb twist and cam lean due to the cable attachment to the cam. While drawing the bow, both the bowstring and cables pulling at their respective points near the center of the axle balance each other to effect a level limb and straight cam alignment.
However, near the end of the draw, letoff is accomplished by transferring the force of pull on the bowstring to the take up and letoff cables which hold the tension at full draw. Therefore in a bow with 80% letoff, 80% of the pull by the bowstring on the axle has been transferred over to the cam pull point on the axle. A problem with the design is that the cable pulling at a point away from the center of the axle causes limb twist and cam lean, resulting in limb stress and inaccuracy.
With a bowstring groove, a cable take up groove, and a cable let out groove, the cable pulls to the right of the axle, causing limb twist and cam lean.
An improvement was accomplished by using only two grooves, one for the bowstring groove, and using only one groove for both take up and let out cables. This results in the cable pulling closer to the center of the axle, reducing limb twist and cam lean. However, as tension was still transferred from the first bowstring groove to the second cable groove upon letoff, limb flex and draw force modules straightness was still negatively affected.
An additional problem with draw force modules is that the friction of the grooves causes string and cable wear and thus reduces the speed of the arrow.
Examples of some prior compound bows employing cam systems are shown in Ketchum (U.S. Pat. No. 3,990,425), Simonds et al. (U.S. Pat. No. 4,401,097), Simonds (U.S. Pat. No. 4,483,753), Miller (U.S. Pat. No. 6,688,295), Darlington (U.S. Pat. No. 6,990,970) and Larson (U.S. Pat. No. 7,441,555) all of which are incorporated herein by reference.